In daily life, most kinds of material as we know, such as rubber and cloth, have positive Poisson's ratios. When a positive Poisson's ratio material is stretched along one direction, the positive Poisson's ratio material will become thinner along another direction that is perpendicular to the stretching direction (as shown in FIG. 1a). In contrast, a negative Poisson's ratio material behaves in the opposite way. When the negative Poisson's ratio material is stretched along one direction, the negative Poisson's ratio material will become thicker along another direction that is perpendicular to the stretching direction (as shown in FIG. 1b). The negative Poisson's ratio material is rare, but such material has better performance in many aspects than the positive Poisson's ratio material, such as the formation of synclastic curvatures under bending (as shown in FIG. 2a, when the negative Poisson's ratio material positioned in a plane is forced, the negative Poisson's ratio material bends along the same perpendicular direction). In contrast, the positive Poisson's ratio material positioned in a plane is forced to bend along one direction, the positive Poisson's ratio material will further bend along an opposite direction and generate saddle-shaped form (as shown in FIG. 2b). The aforementioned property of the positive Poisson's ratio material restricts its application in the aspect of manufacturing outdoor protective clothes. Since many body structures in a human body are all synclastic bend-shaped, the synclastic curvature property of the negative Poisson's ratio material is very useful. When the negative Poisson's ratio material is applied to kneepieces and elbow guard pieces of a suit of outdoor protective clothes, the negative Poisson's ratio material can bend along the same directions as knees and elbows of a human body and form spherical shapes. Thus, the protective clothes can be more flexible in sports. Furthermore, the negative Poisson's ratio material further has excellent properties of preventing concavity and absorbing energy.
Because of the unique performance of the negative Poisson's ratio material, developments of the negative Poisson's ratio have been one of hot researches in the material field since a synthetic negative Poisson's ratio material was first reported in 1987. In recent years, the negative Poisson's ratio effects realized by textile structures, especially by knitted structures, have gained much attention.
U.S. Pat. No. 20110046715A1 and WO. Pat. No. 2009002479A1 demonstrate a series of warp knitted mesh fabrics. A few kinds of the warp knitted mesh fabrics have the negative Poisson's ratio effects. However, these fabrics have bigger meshwork structures, so that the fabrics can merely be in special use rather than be ordinary fabrics. Moreover, elastic recovery of these fabrics is poor, and the negative Poisson's ratio effects of these fabrics may disappear under the action of repeated stretch. These fabrics are difficult to return to their initial state after deformation, thus these fabrics have low practical value.
WO. Pat. No. 2010125397A1 demonstrates another kind of warp knitted fabric with a negative Poisson's ratio. The fabric is made of two kinds of yarns with differences in modulus. The yarn with the lower modulus is used for the ground stitch, and the yarn with the higher modulus is used as the laid-in yarn. A double arrowhead structure (re-entrant quadrilaterals) is formed on the ground stitch to generate the negative Poisson's ratio effects. This kind of fabric generates the positive Poisson's ratio effects when being stretched along the warp direction or the weft direction. Only this fabric is stretched along a diagonal direction, can it generate lower negative Poisson's ratio effects, and the lower negative Poisson's ratio effects can be realized merely under a lower tensile strain range (lower than 10%).
U.S. Pat. No. 20080011021A1 and WO. Pat. No. 2008016690A2 demonstrate a kind of warp knitted fabric with a negative Poisson's ratio, which is similar to the fabric demonstrated in WO. Pat. No. 2010125397A1. This kind of fabric generates the negative Poisson's ratio effects based on re-entrant quadrilaterals formed by triangle mesh structures, but the negative Poisson's ratio effects are also insignificant.
It can be seen from the above patents that knitted fabrics with negative Poisson's ratios in the prior art are usually knitted based on warp knitted structures by using warp knitting technology. The knitted fabrics with negative Poisson's ratios made by these methods are difficult to knit, and the negative Poisson's ratio effects thereof are neither significant nor stable. Moreover, elastic recovery of the aforementioned fabrics is poor, which restricts the application of these fabrics.